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In Lewis' 5th doctrine of possible worlds in modal realism it says:

Possible worlds are unified by the spatiotemporal interrelations of their parts; possible worlds are spatiotemporally isolated from each other.

In other words Possible Worlds cannot have two (or more) distinct space-times. Why is this? Is there a way to redefine Modal Realism to include Possible Worlds with multiple space-times?

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As far as I understand Lewis, the real world is distinguished from all possible worlds solely by the contingent fact that's the world we live in. Hence there are no qualitative differences between all worlds. And I can extrapolate from the characteristics of our world to the characteristics of other worlds.

If a world contains multiple spacetimes then I would consider it several worlds: From the viewpoint of physics a world equals its spacetime. And if spacetime is not connected, then we have several non-interacting components, hence several worlds.

But I am not sure whether my answer meets the point of your question. If not, please write a comment.

  • My question was, why can't a possible world be multiple isolated space-times? Surely a possible world can be a multiverse as long as it's spatio-temporally connected, so why can't it be a multiverse whose universes are spatio-temporally isolated from each other? – lootroot Oct 3 '15 at 19:20
  • @lootroot I consider it a question of definition whether one uses the word world for a multiverse or for a connected component of a multiverse. - Probably, your use of world covers multiverses while my use restricts to a single component. – Jo Wehler Oct 3 '15 at 19:31
  • So why doesn't Lewis' definition include multiple isolated space-times? – lootroot Oct 3 '15 at 19:51
  • @lootroot You may follow the discussion on p.71f in Lewis book On the Plurality of Worlds daalv.free.fr/Master-2011-2012/… – Jo Wehler Oct 4 '15 at 7:32

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